Storms destroy homes as floods swamp NSW

Gale force winds have wreaked havoc across Sydney and the NSW south coast, while around 20,000 people are isolated by floodwaters in the state's north.

The SES received more than 4000 calls for assistance overnight, including around 2000 in Sydney, where 100km/h winds resulted in an asbestos scare.

In the city's southeast, savage gusts extensively damaged the roof of the RSL club in Malabar, causing sheets of asbestos to fly onto nearby homes and roads.

A number of streets were blocked off by Fire and Rescue NSW crews on Sunday, as private contractors were tasked with cleaning up the area.

Residents were told not to panic about air contamination but to contact the NSW Environment Protection Authority if they noticed asbestos near their homes or backyards.

"It was an asbestos roof and to be concentrated like that in an area is of some concern but there is no need for residents to be concerned that the air they breathe is contaminated," Superintendent Paul Bailey told Fairfax Media.

Elsewhere in Sydney's east, locals described wind gusts that felt like "mini tornadoes", which damaged a primary school and tore part of the roof off Fox Studios.

The SES also responded to around 400 calls on the south coast, where Kiama was the hardest hit, SES spokeswoman Becky Collings said.

"We had three homes that were completely written off, and seven others that suffered significant damage," she told AAP.

"We had a lot of trees down and roof damage."

Also on the south coast, seven campers were rescued by a helicopter on Sunday morning from their camping site beside the Clyde River near Ulladulla after they were stranded by rising waters.

It was one of 70 rescues carried out by the SES since the heavy rain and flooding began on Friday.

In the Blue Mountains, west of Sydney, police and paramedics worked together to successfully locate three separate groups of bushwalkers who got lost in the extreme weather conditions.

Meanwhile, around 20,000 people isolated in the state's north - from the north coast to the mid-north coast - could remain cut off for up to a week.

"It really just depends how quickly the water can drain away," Ms Collings told AAP.

She said evacuation orders remained in place on the mid-north coast around the Macleay and Hastings rivers.

Properties at Port Macquarie were flooding on Sunday morning as the Hastings River swelled to its expected peak of 1.8 metres.

The Macleay River at Kempsey peaked at 7m on Sunday near the town's CBD, about 30 centimetres below expectations.

"A lot of the shops and homes have avoided inundation," Ms Collings said, but some parts of the town were under a metre of water on Sunday morning.

Townships on both the upper and lower Macleay remain isolated but were beginning to drop.

With calmer weather in many parts of the state on Sunday, the SES said its focus in the next 24 hours would be on recovery.

"The weather conditions have eased somewhat, so that will mean we can hopefully get through the rest of our jobs," Ms Collings said.

Flood warnings from the Bureau of Meteorology (BOM) remain in place for 16 river systems across the state.

Power supplier Ausgrid said about 6,500 homes remained without power, mainly in Sydney's north and east and on the central coast, while Essential Energy estimated a there were a further 8,000 homes across Greater Western Sydney, the Illawarra, the Southern Highlands and Shoalhaven.

Two people have died in the floods.

On Saturday afternoon, the body of a man was found in his submerged car on a flooded road at Mylneford, about 20km northwest of Grafton.

On Friday, a 17-year-old boy died after he was swept into a drainpipe while collecting golf balls in waist-deep water in the town of Kew, near Port Macquarie.